1. Field of the Invention
This invention relates to zoom lenses, and, more particularly, to zoom lenses of which focusing is performed by moving a lens group constituting part of the image forming section in the rear of the zoom section.
2. Description of the Prior Art
Conventionally, in the field of semi- and super-telephoto lenses, what is called "rear" or "inner" focus, that is, the focusing method by moving other than the first lens group, counting from the front has been employed in order to effect focusing easily and quickly. These focusing methods were designed to overcome the drawbacks which were encountered when other generally adopted methods of focusing such as by moving the entire lens system or by moving the front lens members, were applied to lenses of relatively long total length and heavy weight. This was done because a great increase in the driving torque of the helicoid or other operating mechanism must be called for, or because the center of gravity of the lens system changes its position as focusing is performed. Such focusing provision has generally been made at one of the lens groups which lies relatively nearer to the film plane and is of small diameter.
The use of this rear focusing method was advantageous in that it is made easy to reduce the total focusing movement as far as possible without involving any loss in the imaging performance by giving the focusing lens group a proper refractive power and optical arrangement.
Particularly in application to auto-focus cameras, this produces many great advantages in that (a) by virtue of the small-sized and light-weight focusing lens group, focusing can be operated without unduly large stress, in that (b) since the focusing lens group takes its place relatively near the camera body, it is easy to establish a reliable signal transmission between the operating mechanism and a control mechanism in the camera body, and in that (c) because the total focusing movement can be minimized, feedback is easily provided in the in-focus position detecting system.
Similarly, in the field of art of zoom lenses, the above-stated advantages of the rear focusing method are valid. In particular, with regard to the aims of achieving great increases in the zoom ratio and relative aperture, the use of the front focusing method results in an increase in the total length of the lens system, and also an increase in the diameter of the front or focusing lens members.
As a conventional means for improving such problem, a proposal making use of the rear focusing method in the zoom lens is made in Japanese Laid-Open Patent Application No. Sho 57-78513. This approach is that instead of making the focusing provision at the front lens members as had so far prevailed in the prior art, focusing is performed by a lens group constituting part of the image forming section which remains stationary during zooming, while the first lens group in the entire lens system is left stationary during zooming and focusing.
Another proposal in view of extending the zooming range, and minimizing the bulk and size of the entire lens system, by Japanese Laid-Open Application Nos. Sho 53-34539 and 54-25747 provides that the first lens group of positive power is otherwise made to move forward as zooming is effected from the wide angle to the telephoto position, thus increasing the rate of change of image magnification of the second lens group as the variator.
Since, however, the distance from the film plane to the first lens group varies with variation of the focal length of the entire system, the required amount of movement of the focusing lens for the same object distance measured from the film plane is caused to differ with focal length. This implies that after the in-focus condition has been established at a certain focal length, when zooming is allowed to proceed, the image plane will shift. Also in the zoom lens of the type in which the first lens group moves forward when zooming from the wide angle to the telephoto position, the effective diameter of the front lens members which is determined by the off-axis oblique pencil of light rays for the telephoto position is necessarily increased. Since focusing is effected down to shorter object distances, if the front member is selected for focusing purposes, a further increase in the effective diameter of the front lens member is called for.
To overcome these drawbacks, adoption of the rear focusing method in the zoom lens of the character described is considered. Focusing of the zoom lens is performed by a rear lens member, the focusing movement of the rear lens member from a position for an infinitely distant object to a position for a given object distance varies inconveniently as a function of the focal length of the entire lens system. Therefore, a change in the focal length as sensed by a suitable method has to be factored into the required amount of movement of the rear lens member by a computer or by an operative connection between the zoom actuator and an auto-focus apparatus of the TTL type. As a matter of course, it is also possible to adopt this method even in the zoom lens having the focusing provision at the front member. Since, however, the front or focusing lens member is of large size and heavy weight and because the structure of the operating mechanism becomes complicated, it is recommended that such factoring system is advantageously applied to the rear focusing method. It is further to be noted that a remarkable reduction in the diameter of the front member can be achieved since the front lens member remains stationary during focusing.
For use of the zoom lens employing the rear focusing method, mention may be made of the disclosures of U.S. Pat. Nos. 3,391,973, 4,043,642 and 4,054,372.